The other 498 times Howard tells that story he calls Fred homophobic for not changing in front of him in the hotel room.
I guess Fred was uncomfortable with Howard staring at his unclipped Latvian cock...

Well, the scene of him not having a problem climbing into the bath tub with Howard in the movie was just tossed aside as a joke. Wasn't Allison already paranoid/suspecting Howard of having an affair at that time? What if them sharing a tub wasn't the first time?